A lens hood’s main purpose is to block light. If the lens is a prime lens (fixed focal length, non-zooming) the hood will resemble a tube, often larger at one end than the other. For zoom lenses the hood will have a curved opening at one end. This curve is cut to the zoom range of the lens and allows for the wider field of view afforded smaller focal lengths, while still attempting to block most light at a longer focal length. It’s a compromise that matches the compromise of a zoom lens.